Keystone correction apparatus



Sept. 3, 1946. J. A. BUCKBEE KEYSTONE CORRECTION APPARATUS Filed: Feb.14, 1944 FIG.4

I INVENTOR JOHN A. BUCKBEE ATTORNEY Patented Sept. 3, 1946 KEYSTONECORRECTION APPARATUS John A. Buckbee, Fort Wayne, Ind., assignor toFarnsworth Television and Radio Corporation, a corporation of DelawareApplication February 14, 1944, Serial No. 522,311

3 Claims.

This invention relates to magnetic field prc ducing devices, andparticularly to a scanning yoke for a cathode ray tube to produce anelectromagnetic field of a predetermined non-uni form intensity.

According to conventional practice certain types of cathode ray devicesemployed in tele vision systems require compensation for de ficienciesinherent in these devices produced by certain structural limitationsthereof. One device of this character is an iconoscope used as atelevision camera tube wherein, in order to avoid obstruction of theoptical system, the electron gun for producing the scanning beam ofelectrons is disposed angularly with respect to the photosensitiveelectrode to be scanned. By reason of this angular relationship, in theabsence of suitable compensating means, the area of the photosensitiveelectrode scanned by the beam has a trapezoidal or keystone shape. Anumber of proposed compensating arrangements to overcome this scanningdeficiency comprise systems for deflecting the scanning beam by means ofnon-uniform electromagnetic fields having divergent intensitydistribution characteristics. In order to produce a field of thischaracter it has been proposed to suitably form opposed salient polepieces of a deflecting yoke, about the legs of which the windings areplaced. It also has been proposed to provide a greater number of windingturns upon one of two opposed salient pole pieces than upon the other.Another prior art expedient has been to concentrate a greater number ofcoil conductors in one portion of a cylindrically shaped coil than inothers.

As is well known in the art, however, deflecting coils of these typesare relatively inefiicient for the reason that a large percentage of thefields produced thereby are disposed exteriorly of the tube andcontribute nothing toward the deflection of the electron beam within thetube. As a consequence, a substantial portion of the energy supplied tothe coil is dissipated without being gainfully employed. Furthermore,there is a tendency produced by the exterior portion of the field tocreate undesired electrical disturbances in other equipment associatedwith cathode ray tubes employed for conventional purposes, such ascamera tubes in television systems.

It is an object of this invention, therefore, to provide a novelscanning yoke for a cathode ray tube which is characterized by itsability to produce in a highly efiicient manner an electromag-.

netic field which has a predetermined non-uniform intensity distributionthroughout a plane substantially normal to the path of the cathode ray.

In accordance with this invention, there is pro- Vided for use with acathode ray tube an annular core which completely surrounds a portion ofthe tube through which travels the electron beam. There is wound aboutthe core a coil array having a greater number of turns about one portionof the core than about others, whereby to produce within the tube spaceenveloped by the core a substantially unidirectional electromagneticfield having a predetermined divergent intensity distribution.

More particularly, there is wound about this core a pair of separatecoils forming at least a portion of a toroid. The placing of the coilsupon the core and the electrical interconnection thereof to a source ofunidirectional energy is such that adjacent ends of the coils producemagnetic poles of like polarity. The ends of the two cells which areadjacent are relatively closely spaced on the core While the other endsof the coils are relatively widely spaced on the core. In this mannerthe field in the vicinity of the closely spaced coil ends is relativelystrong while the field adjacent the Opposite portion of the core isrelatively weak. The structural arrangement of the coils upon the coreis of such a na ture that there is produced within the portion of thetube enveloped by the core, a non-uniform field which has apredetermined divergent intensity distribution characteristic.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following description,taken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is an elevational view in cross section of a cathode ray tubeprovided with a scanning yoke in accordance with this invention;

Fig. 2 is a sectional view of a portion of the cathode ray tube taken onthe line 22 of Fig. 1;

Fig. 3 is a schematic diagram illustrating one manner of winding thecoils on the core and the electrical interconnection thereof; and

Fig. 4 is a diagram illustrating the field distribution within the tubespace enclosed by the scanning yoke illustrated in Figs; 1 and 2.

Referring now to the drawing and particularly to Fig. 1, there isprovided, in accordance with this invention, a cathode ray tube ll ofconventional construction. This tube is a television camera tube of theiconoscope type and includes an electron gun assembly 12 for generatingan electron beam with which to scan a mosaic electrode I3. The electrode13 is mounted within the main body portion of the tube and is disposedin a plane which is substantially perpendicular to the axis of aconventional optical system (not shown) by means of which lightreflected from the television subject is projected upon the mosaic. Theelectron gun structure [2 is located in the tubular neck portion of thetube which is angularly disposed with respect to the mosaic [3 in ordernot to interfere with the optical system. The neck portion of the tube His enveloped, in the region between the electron gun assembly [2 and themain body portion of the tube, by a vertical scanning yoke l4 and ahorizontal scanning yoke [5.

The vertical scanning yoke I4 is placed nearer to the electron gunassembly than the horizontal scanning yoke i5 in order that the electronbeam may be suitably deflected in a manner to be described. Theconstruction of the two scanning yokes is similar, and in general is inaccordance with the structure disclosed in a copending application ofJohn A. Buckbee, Serial No. 519,719, filed January 26, 1944, andentitled Scanning and focusing yoke. Therefore, only the horizontal yokeembodying the instant invention will be described in detail by referringto Figs. 1 and 2. The yoke 15 consists of a ferromagnetic core 16 whichpreferably is of a laminated structure formed by winding a continuousribbon of transformer iron to form a laminated ring of approximatelyone-eighth inch thickness, for example. A pair of coils I7 and i8 may bewound about the core H5 in such a manner that together they extendalmost completely around the core. These coils may be wound identicallyand placed opposite to one another on the core, in which case they areelectrically connected in parallel. Alternatively, they may be woundoppositely to one another and also placed oppositely to one another onthe core, in which case they are electrically connected in series. It isthe latter arrangement which is illustrated herein, and the seriesconnected coils are connected to a source of unidirectional energy asindicated in Fig. 3. In either type of arrangement however, there areproduced by the respective coils magnetic poles of like polarityadjacent to one another. For example, at the lower portion of theannular core it, as viewed in Fig. 3, there are produced magnetic northpoles while at the upper portion of the core magnetic south poles areproduced. It will be obvious to those skilled in the art that the sameresults may be obtained by other coil arrangements together withappropriate electrical interconnections thereof.

In addition to the coils l1 and I8 there also is provided at the lowerportion of the core 86 a pair of auxiliary deflection coils l9 and 2|.These coils may be wound and arranged upon the core similarly to thecoils l1 and !8 so that there are produced adjacent the lower portion ofthe core magnetic poles of like polarity which cor responds to thepolarity of the magnetic poles produced in this portion of the core bythe windings l1 and I8. As illustrated, the auxiliary coils I9 and 2|are wound about the exterior of the coils IT and 18. However, it iscontemplated to be within the scope of this invention that either theauxiliary coils be placed adjacent to the core with the coils l1 and I8wound about the auxiliary coils, or the two sets of coils may be placedupon the core with the respective turns thereof intermingled, ifdesired. In any case the auxiliary coils l9 and 2| extend for onlyrelatively short distances around the annular core l5, as indicated inthe drawing. Alternatively, a single pair of coils may be employed, eachcoil having a distributed winding wherein a greater number of turns isconcentrated at one end or the coil than at the other. In some casesonly coils similar to the coils l9 and 2| may be sufficient to produce afield of the desired strength and intensity distribution.

The electrical interconnection of the auxiliary coils may or may notcorrespond to the interconnections of the coils I1 and I8, dependingupon whether 01' not the winding of the auxiliary coils is the same asor different from the coils I1 and 18. The auxiliary coils. areconnected to a conventional source (not shown) of unidirectionalsawtooth scanning energy which, if desired, may be the same source whichsupplies the coils I! and H3 in which case the two sets of windings maybe connected either in parallel, as shown, or in series to the source ofenergy.

Fig. 3 illustrates diagrammatically one manner in which the coils may bewound upon the annular core It. As illustrated, the coils I! and I8 andalso the auxiliary coils l9 and 2| are wound oppositely upon the coreand consequently are connected in series as indicated in the lowerportion of 3. The other ends of the respective windings are connected,as shown, in parallel to a suitable source of unidirectional energy.

Referring now to the operation of the described apparatus embodying thisinvention, it is assumed that a concentrated beam of electrons isgenerated by the electron gun assembly l2, and is directed toward themosaic electrode l3. In the tube space enveloped by the verticalscanning yoke [4 there is produced an electromag netic field ofsubstantially uniform intensity which extends from the lower to theupper parts of the neck of the tube II. The magnitude of this field isvaried in a well known manner under the control of a suitable scanninggenerator connected to the yoke l4. Under the influence of the verticaldeflecting field thus produced the electron beam is deflected at arelatively slow rate so as to scan the mosaic electrode I3 from top tobottom, for example.

In the space of the tube neck enveloped by the horizontal scanning yokel5 there is produced by means of the combined action of the coilsl'l--l8 and i92l an electromagnetic field which has an intensitydistribution which varies from one point to another in a predeterminedmanner. The general character of the field distribution is illustratedgraphically in Fig. l. It is seen that the field adjacent the lowerportion of the core I6 is relatively strong, whereas, adjacent the upperportion of the core the field is relatively weak. The magnetic lines offorce comprising this field diverge from a central line extendingthrough the opposite ends of the coils i! and I8. It is to be noted thata field having such a divergent characteristic is produced by theauxiliary coils l9 and 2|, by reason of the positions in which they aremounted on the core IS. The field produced by the coils IT and I8 issubstantially uniform throughout the space enveloped by the core, andthe lines of force comprising this field extend substantially parallellyfrom the lower to the upper portions of the core. It is apparent,therefore, that there is produced by means of the two sets of coils acomposite electromagnetic field which is characterized by the describeddivergence of the lines of force. 7

The magnitude of this composite divergent field is varied in a wellknown manner by suitable connection of the coils to a source ofdefiection voltage. Under the influence of the varying magnitude of thedivergent horizontal scanning field, the electron beam is deflectedhorizontally from one side of the mosaic electrode [3 to the other.However, by reason of the divergent character of this field, theelectron beam when directed through the upper portion of the horizontalscanning field is not deflected horizontally as much as it is whentraversing the lower portion of the divergent field. In the present casethe compensation afforded by the divergent field produced in accordancewith the present invention is sufficient in magnitude to cause theelectron beam to traverse a substantially rectangular area of the mosaicl3.

It will be noted that, while not specifically illustrated herein, ameans for focusing the electron beam will be required. Inasmuch as it iscontemplated that such a structure may be conventional and forms no partof the present invention, it has been omitted from the drawing in orderto simplify the illustration of the apparatus embodying the invention.If desired, the focusing of the electron beam may be effected by meansof a coil Wound on either or both of the scanning yokes l4 and 15. Thefocusing coil may be Wound either externally or internally of thescanning coils.

While there has been described what, at present, is considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and therefore, it is aimedin the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What is claimed is:

1. An electromagnet deflecting system for a cathode ray tube comprising,an annular ferromagnetic core disposed about the electron path of saidtube, a first deflecting coil wound about said core in a manner toproduce within the space enclosed by said core a unidirectionalelectromagnetic field of substantially uniform intensity distribution,and a second deflecting coil wound similarly to said first coil aboutonly a portion of said core in a manner to produce within the spaceenclosed by said core a unidirectional electromagnetic field ofnon-uniform intensity distribution, said two fields being similarlypoled and combining to form a composite unidirectional electromagneticfield of divergent intensity distribution.

2. An electromagnetic deflecting system for a cathode ray tubecomprising, an annular ferromagnetic core disposed about the electronpath of said tube, a first deflecting coil comprising two portionselectrically interconnected and wound about said core in a manner toproduce Within the space enclosed by said core a unidirectionalelectromagnetic field of substantially uniform intensity distribution,and a second deflecting coil symmetrically disposed with respect to saidfirst coil and comprising two portions electrically interconnected andwound similarly to said first coil about only a portion of said core ina manner to produce within the space enclosed by said core aunidirectional electromagnetic field oi non-uniform intensitydistribution, said two fields being similarly poled and combining toform a composite unidirectional electromagnetic field of divergentintensity distribution.

3. An electromagnetic deflecting system for a cathode ray tubecomprising, an annular ferromagnetic core disposed about the electronpath of said tube, a first deflecting coil comprising two portionselectrically connected in series and wound in opposite directions aboutsaid core to produce within the space enclosed by said core aunidirectional electromagnetic field of substantially uniform intensitydistribution, and a second defleeting coil symmetrically disposed withrespect to said first coil and comprising two portions electricallyconnected in series and wound similarly to said first coil in oppositedirections about only a portion of said core to produce within the spaceenclosed by said core a unidirectional electromagnetic field ofnon-uniform intensity distribution, said two fields being similarlypoled and combining to form a composite unidirectional electromagneticfield of divergent intensity distribution.

JOHN A. BUCKBEE.

